The long-term objective of studying the hepatitis B family of viruses is to understand the phenomenon of viral pathogenesis. An important aspect of the overall problem is to characterize the molecular biology of virus replication, including target-cell specificity, mechanisms of initiation of infection, and the nature of control of expression of viral gene products. We intend to study these problems using duck hepatitis B virus. Duck hepatitis B virus (DHBV) is a new member of the hepatitis B family of viruses. The virus establishes a chronic infection of domestic ducks as a consequence of vertical transmission through the egg. The ready availability of viremic ducks provides an ideal system in which to study the replication of a hepatitis B-like virus. The specific aims of our proposal are to characterize the routes of natural vertical transmission of DHBV, to establish conditions for experimental transmission leading to acute or chronic infections, and to characterize the molecular mechanisms of DHBV replication in tissues of infected ducks. Towards the latter goal we have cloned the DHBV-genome into bacteriophage vectors and used the cloned DNA to prepare nucleic acid hybridization probes to begin an analysis for viral replication in organs of viremic ducks. This analysis has already shown that virus infection is most prominent in the liver, but that a limited amount of virus growth also occurs in the pancreas. We plan to use probes for viral nucleic acids and proteins to further characterize virus expression in tissues of infected ducks, and to begin an analysis for virus growth in fibroblast and hepatocyte cultures derived from Pekin ducks. Immunofluorescence microscopy will be applied to sections of organs from viremic ducks in order to further identify cell types in which virus replicates.